The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Catalytic or combustible (flammable) gas sensors have been in use for many years to, for example, prevent accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases. As illustrated in FIGS. 1A and 1B, a conventional combustible gas sensor 10 typically includes an element such as a platinum heating element wire or coil 20 encased in a refractory (for example, alumina) bead 30, which is impregnated with a catalyst (for example, palladium or platinum) to form an active or sensing element, which is sometimes referred to as a pelement 40, pellistor, detector or sensing element. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987). Combustible gas sensors are also discussed generally in Firth, J. G. et al., Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J. G., Eds., Detection and Measurement of Hazardous Gases, Heinemann, Exeter, 29 (1981).
Bead 30 will react to phenomena other than catalytic oxidation that can change its output (i.e., anything that changes the energy balance on the bead) and thereby create errors in the measurement of combustible gas concentration. Among these phenomena are changes in ambient temperature, humidity, and pressure.
To minimize the impact of secondary effects on sensor output, the rate of oxidation of the combustible gas may be measured in terms of the variation in resistance of sensing element or pelement 40 relative to a reference resistance embodied in an inactive, compensating element or pelement 50. The two resistances are typically part of a measurement circuit such as a Wheatstone bridge circuit as illustrated in FIG. 1C. The output or the voltage developed across the bridge circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The characteristics of compensating pelement 50 are typically matched as closely as possible with active or sensing pelement 40. Compensating pelement 50, however, typically either carries no catalyst or carries an inactivated/poisoned catalyst.
Active or sensing pelement 40 and compensating pelement 50 can, for example, be deployed within wells 60a and 60b of an explosion-proof housing 70 and can be separated from the surrounding environment by a flashback arrestor, for example, a porous metal frit 80. Porous metal frit 80 allows ambient gases to pass into housing 70 but prevents ignition of flammable gas in the surrounding environment by the hot elements. Such catalytic gas sensors are usually mounted in instruments which, in some cases, must be portable and, therefore, carry their own power supply. It is, therefore, desirable to minimize the power consumption of a catalytic gas sensor.
Catalytic combustible gas sensors are typically used for long periods of time over which deterioration of the sensing element or the like and malfunction of circuits may occur. A foreign material such as an inhibiting material or a poisoning material (that is, inhibiting or poisoning of the catalyst of the sensing element) may, for example, be introduced to the sensing element. An inhibiting material typically will “burn off” over time, but a poisoning material permanently destroys catalytic activity of the sensing element. In general, it is difficult to determine such an abnormal operational state or status of a combustible gas sensor without knowingly applying a test gas to the combustible gas sensor. In many cases, a detectible concentration of a combustible gas analyte in the ambient environment is a rare occurrence. Testing of the operational status of a combustible gas sensor typically includes the application of a test gas (for example, a gas including a known concentration of the analyte or a simulant thereof to which the combustible gas sensor is similarly responsive) to the sensor. Periodic testing using a combustible gas may be difficult, time consuming and expensive.
For decades sensor designers have been perplexed with the problems of contamination and/or degradation of their catalyst structures. Sulfur compounds (inhibitors) have been known to inhibit the catalyst structures, and filtering techniques are used to prevent their passage into the structure. If they do enter the structure, they are bound until a sufficient level of heat is applied to promote their release or decomposition. Volatile silicon compounds (poisons) are also known to cause significant issues with catalytic structures as they are permanently retained, and eventually result in the total inactivity of the catalyst. Finally, high levels of hydrocarbons can also deposit incomplete and/or secondary byproducts such as carbon within the structure.
All of these issues go undetected by the high sensitivity bridge circuits used over the years in combustible gas sensors. Users have long reported cases where their catalytic sensors are reading zero (that is, the bridge circuitry is balanced), yet they show little response to gas challenges. A number of sweeping, ramping and pulsing techniques have been attempted to detect minute changes in the thermodynamic properties of the sensing elements. However such techniques are only partially effective when large scale changes have occurred. Moreover, the sensors have to be taken off-line for analysis to use these techniques, potentially missing a dangerous safety event.